1. Field of the Invention
The present invention relates to heat transfer tubes which are utilized as vaporization and condensation tubes in apparatus such as heat exchangers and heat pipes.
2. Background Art
Heat transfer tubes made of metals, such as copper, having many straight or helical grooves on the inner surfaces, which can be manufactured by roll-forming or drawing processes, have been known in the past.
These grooves provide the following benefits;
1. When used as condensation tubes, these heat transfer tubes produce improved liquefaction efficiency by increasing the turbulence of the vapors as well as improved nucleation of the liquid phase brought about by the action of the surface irregularities. Furthermore, the surface tension effects on the liquid in the grooves serve to retain the fluid and promote good drainage, leading to increased reflux efficiency. PA0 2. When these tubes are used in vaporizers, the edges of the grooves act as nucleation sites for the bubbles to provide rapid boiling, thus increasing the efficiency of liquid to vapor conversion. Furthermore, the surface tension effects serve to distribute the vaporizing liquid evenly throughout the vaporizer, promoting efficient conversion.
To improve the performance of such heat transfer tubes, it is advisable to decrease the width of the inner edges of the groove, making its profile resemble a trapezoid. Such a trapezoidal or pear-shaped grooves will promote nucleation of bubbles on the interior of the groove, which would act as nuclei for the formation of vapors, thus leading to a more efficient boiling and vaporization process. Also, the surface tension forces within the groove can be utilized more effectively to improve the liquid transport efficiency, leading to an overall gain in the heat transfer efficiency.
However, the conventional mechanical processes of manufacturing single grooved heat transfer tubes can only produce groove profiles whose opening is wider than that of the bottom or the outside edge. It has not been possible to manufacture tubes whose profile is pear-shaped, when viewed in the direction of the tube axis, and consequently, there was a limitation in improving the heat transfer performance of heat transfer apparatus such as heat exchangers.